Such an arrangement for controlling the internal potential is disclosed in German patent application DE-OS No. 27 55 050. In the arrangement described therein, a polyester made conductive by carbon black particles is wound around a wiring assembly composed of insulated conductor elements and d-c coupled at a single contact point with one of the conductor elements, this conductor element being free of its insulation in the range of its contact point. A disadvantage of such a structure is that due to the relatively great resistance of the semi-conductive web, a uniform potential over the entire rod length is not possible. Beyond that, the conductivity, which is possibly influenced by the processing technique can not be determined in advance, because the fibrous web can be influenced by surrounding resins. On the other hand, the provision of several contact points between the semi-conductive band and the conductor element, would be too expensive, particularly since the conductor element constantly changes its position in the wiring assembly by the twisting, so that it is difficult to find for the application of the contact points.
In another arrangement disclosed in patent application of Austria AT-PS No. 277 375, the internal potential is controlled by a band of juxtaposed metallic threads, which are woven with each other by a weft of non-conductive material. This band is likewise d-c coupled at a single contact point with one of the conductor elements. Due to the relatively low resistance of the band, the potential is uniform over the entire rod length, but because of the low resistance there are high eddy current losses, which lead to undesired heating. This heat can become so great in the case of a short circuit that the metallic threads of the band melt.